1. Field of the Invention
This invention relates to a semiconductor device, specifically to a semiconductor device provided with a clamp diode.
2. Description of the Related Art
A Zener diode has been used as a diode to protect a gate insulation film of a MOS transistor by clamping a voltage applied between a drain (or a source) and a gate of the MOS transistor. A structure of the Zener diode is described referring to FIG. 4. As shown in the figure, an N− type semiconductor layer 51 is formed on a P type semiconductor substrate 50, and a P+ type diffusion layer 52 is formed in a surface of the N− type semiconductor layer 51. Also, an N+ type diffusion layer 53 is formed in a surface of the P+ type diffusion layer 52.
An insulation film 54 is formed to cover the surface of the N− type semiconductor layer 51. There is formed a cathode electrode 55 connected with the N+ type diffusion layer 53 through a contact hole that is formed in the insulation film 54 on the N+ type diffusion layer 53. Also, there is formed an anode electrode 56 connected with the P+ type diffusion layer 52 through a contact hole that is formed in the insulation film 54 on the P+ type diffusion layer 52.
In a structure described above, the N+ type diffusion layer 53 serves as a cathode, while the P+ type diffusion layer 52 serves as an anode. When a reverse bias voltage is increasingly applied between the cathode electrode 55 and the anode electrode 56, there is caused a breakdown in a PN junction formed of the N+ type diffusion layer 53 and the P+ type diffusion layer 52. The reverse bias voltage at the breakdown is called a Zener voltage.
The gate insulation film of the MOS transistor is protected by clamping the voltage applied to the gate of the MOS transistor using the Zener diode.
A MOS transistor circuit using the Zener diode is described in Japanese Patent Application Publication No. 2002-84171.
A breakdown voltage of a clamp diode which clamps the voltage applied to the gate of the MOS transistor needs to be set lower than a withstand voltage of the gate insulation film of the MOS transistor. The withstand voltage of the gate insulation film means either an intrinsic breakdown voltage at which a dielectric breakdown of the gate insulation film occurs, or a TDDB (Time Dependent Dielectric Breakdown) voltage (An electric field in the gate oxide is 4 MV/cm.) which takes a change in the withstand voltage over time into consideration. The breakdown voltage of the clamp diode is preferably set lower than the TDDB voltage.
When the gate insulation film is reduced in thickness, the breakdown voltage of the clamp diode needs to be reduced accordingly since the withstand voltage of the gate insulation film is also reduced. On the other hand, the breakdown voltage of the Zener diode is determined in relation to an impurity concentration in the PN junction. Therefore, the breakdown voltage of the Zener diode can be reduced by increasing an impurity concentration in the P+ type diffusion layer 52 or an impurity concentration in the N+ type diffusion layer 53.
When the impurity concentration in the P+ type diffusion layer 52 or the impurity concentration in the N+ type diffusion layer 53 of the conventional Zener diode is increased, however, there is a problem that a leakage current is increased, although the breakdown voltage is reduced.